The present invention relates to a process for the preparation of mono-esters of aliphatic, saturated or unsaturated, straight or branched chain monocarboxylic acids having from 2 to 24 carbon atoms and polyhydric alcohols in the presence of an acyl hydrolase.
When a carboxylic acid and a polyol are contacted for an esterification process, a mixture is formed comprising fully esterified polyol and polyols partially esterified to various degrees: monoesters, diesters etc. It has appeared very difficult to obtain selectively mono-esters when various hydroxyl groups on the polyol are available for reaction.
There have been many investigations in the past for developing a convenient method for the selective preparation of mono-esters of polyols. Both enzymatically catalysed as well as non-enzymatically catalysed methods have been investigated. A review of the various ways in which this subject was investigated using enzymatically catalysed processes is given by U. T. Bornscheuer in xe2x80x9cEnzyme and Microbial Technologyxe2x80x9d, 17, 578-586, 1995.
An example of such a process, using enzymes having esterase activity, such as lipases or esterases, is given in European Patent Specification EP-B-0,215,038 (Novo Industri A/S). In this patent a process for the preparation of monoglycerides has been described in which first two hydroxyl groups of glycerol are blocked by converting them into a ketal or an acetal, such as isopropylidene glycerol or glycerol diethylketal. This ketal or acetal is then reacted with a carboxylic acid or a carboxylic acid ester in the presence of an esterase. The acetal or ketal protecting group is then removed by acid catalysis from the resulting ester to produce monoglyceride. This synthesis constitutes a rather cumbersome route, however, in that first two adjacent hydroxyl groups of the glycerol molecule need to be blocked involving chemical reaction and the blocking group has then to be removed in a final deprotection step.
Therefore, there is still a need for a simple enzymatic process for preparing fatty acid mono-esters of polyhydric alcohols which is economically attractive with regard to the price of the enzyme and which leads to high yields of monoglyceride and the smallest possible amount of di-esters or higher esters.
A group of closely related glycoproteins, known as patatin, is responsible for lipid acyl hydrolase activity found in potato tubers. The lipid acyl hydrolase is only known for its activity to catalyze the deacylation (hydrolysis) of a range of naturally occurring lipids, e.g. monoglycerides, diglycerides and phospholipids (Biochem. J. 121 (3), 379-390 (1971)).
The use of a lipid acyl hydrolase for the formation of wax esters from long chain monocarboxylic acids and long chain monohydric alcohols has been demonstrated (S. Dennis and T. Galliard, Phytochemistry 13 [11], 2469-2473 [1974]). It is surprising that the synthesis of polyol mono-esters, such as monoglycerides, has never been suggested or proposed.
We have found that a lipid acyl hydrolase occurring inter alia in potato tubers, is particularly suitable for the enzymatic production of mono-esters of aliphatic carboxylic acids and polyhydric alcohols. This enzyme is available in good quantities, because it can relatively easily be obtained from abundantly available raw materials. Although the tubers contain the highest amount of said enzyme, lesser amounts can also be found in other parts of the potato plant. The enzyme can also be obtained by applying genetic engineering techniques.
Therefore the present invention relates to a process for the preparation of mono-esters of aliphatic, saturated or unsaturated, straight or branched chain C2-C24 monocarboxylic acids and polyhydric alcohols in the presence of an enzyme, wherein the enzyme is potato lipid acyl hydrolase, which means that the hydrolase is obtainable from potatoes or is identical in substrate specificity. Said enzyme selectivity catalyzes the formation of mono-glycerides. Higher esters are formed too, but in very small quantities only.